Nickel-based olefin oligomerization and polymerization initiators are of interest in industrial and academic laboratories [1,2]. Cationic versions are the most commonly encountered [3]. Neutral species, although less active, are under investigation because of their higher tolerance toward functionalities [4, 5, 6, 7]. Zwitterionic counterparts are the least common and provide an intermediate range of reactivities [8].
While developing tandem catalytic processes [9, 10, 11, 12], one of the inventors discovered that the reactivity of SHOP-type catalysts such as [(C6H5)2PC6H4C(O)O-κ2P,O]Ni(η3—CH2CMeCH2) [13] increases upon addition of B(C6F5)3 [14,15]. Carbonyl coordination to the borane gives zwitterionic [(C6H5)2PC6H4C(O—B(C6F5)3)O-η2P,O]Ni(η3—CH2CMeCH2) and removes electron density from nickel. Examination of ligand/reactivity relationships for cationic Ni(diimine) initiators [3,16] led to the design and synthesis of {(H3C)C[═NAr]C[O—B(C6F5)3][═NAr]-κ2N,N″}Ni(η3—CH2C6H5), with which high molecular weight polyethylene (PE) can be produced for bulky aromatic (Ar) substituents [17]. Several other ligand types have been reported that are amenable to the concept of Lewis acid attachment on a ligand site to redistribute electron density, including 3-(1-arylimino-ethyl)-acetylacetonato [18], 2-diphenylphosphinylbenzamido [19], N-(2-benzoylphenyl)benzamido [20], α-iminoenamido [21], 2-(alkyldeneamino)benzoato [22] and iminoamido pyridine [23]. However, none of these systems has been able to produce high molecular weight PE, in the absence of bulky Ar substituents, or polypropylene.